Abstract
As part of a trend towards diversification of cultivated areas, varietal mixtures are experiencing a revival of interest in agriculture. Varietal mixtures are particularly at the origin of a higher production, a reduction of chemical inputs and improve the sustainability of our agricultural system. Moreover, they allow a better management of diseases. In addition to the epidemiological effects of varietal mixtures on the spread of pathogens, little is known about the effect of intraspecific interactions between plants and their impact on plant responses to pathogens. In this thesis, rice or durum wheat genotypes from different populations with different selection histories were grown with different conspecific neighbors. In total, more than 402 pairwise mixtures were manually inoculated under controlled conditions to prevent pathogen spread. We measured the susceptibility of the mixtures to different foliar pathogens and estimated the factors that may play a role in the modification of susceptibility by the intraspecific neighbor. The responses of plants to their neighbors were explored by transcriptomic and physiological analysis. In addition, we investigated genetic determinants by conducting an association genetics experiment and identified a locus in the neighboring genotype that can trigger susceptibility modification in neighboring plants. Our study provides evidence that disease reduction in varietal mixtures can also result from direct plant interactions, in addition to mechanisms affecting pathogen spread at the field level. This phenomenon, called 'Neighbour-Modulated Susceptibility' (NMS) provides essential information in agronomic mixture assembly.